Surfaces of laminates or components composed of fibers such as glass fibers, carbon fibers, or plastic fibers in a matrix of curable resins such as epoxy resins, unsaturated polyester resins, or vinyl ester resins generally feature less acceptable surfaces, which, moreover, are not resistant to light and weathering. These must first be treated with protective coatings if these components are to be employed in applications for which decorative or weather-resistant surfaces are required.
Components' surfaces are as a rule painted with suitable coating materials, predominantly weather-resistant and anticorrosive paints, for example, those based on aliphatic polyurethanes. To ensure sufficient adhesion of the coating to the component, the surfaces to be painted must, however, first undergo complex pretreatment. Surfaces ready for painting are usually obtained following a number of process steps. The surface of the demolded component is first sanded to effect a complete removal of any mold releasing agents. The surface is then coated or smoothed with a filling compound to level out any surface defects such as pores or individual protruding fibers exposed by the sanding treatment. Once the filling compound has hardened, the surface is again sanded to obtain a smooth surface ready for painting.
One alternative to this time-consuming and laborious pretreatment process is the application of a gelcoat. A gelcoat is a composition based on a resin system and is applied to the surfaces of components using an in-mold composite construction process. The use of gelcoats yields smooth surfaces during the manufacturing process of the component, which surfaces are ideal for sanding. The surfaces may then be painted immediately following sanding. The gelcoat is generally placed in a component mold as the first layer, which is then pre-cured or incipiently gellated to an extent at which the dryness grade 6 in accordance with DIN 53 150 has been reached, at which it complies with the mechanical requirements for subsequent processing. Fibers, for example, in the form of woven fabrics, non-woven fabrics, or laid webs, and the laminating resin containing the thermosetting resin employed as a matrix are then placed on the partially gellated gelcoat film. The entire composition is then hardened to completion. The gelcoat film must be sufficiently stable that the fibers can be applied, and where necessary removed again, without damaging the film. In the case of extremely large molds, such as rotor blades for wind turbines, woven fabrics or non-woven fabrics are usually applied by hand. It must consequently also be possible to walk on the gelcoat film without the film sustaining damage.
Use has until now been made of filled gelcoats. The filler distributed in the resin system forms a framework which provides the required mechanical stability with only minimal precuring or only after minimal progression of the curing reaction of the gelcoat. The use of transparent gelcoats is more advantageous since laminating flaws, such as gas bubbles or dry areas in the laminate which have not been coated by resin, can be readily detected and repaired following removal of the component from the mold. Transparent gelcoats not containing filler require a much more intense curing process to 0 achieve the required stability of the film. More intense progression of the curing reaction, however, results in considerably shorter laminating times. The laminating time is taken to be the amount of time occurring between the point in time at which the gelcoat placed in the mold becomes tack-free and the point in time at which the gelcoat film must be sufficiently laminated to provide adhesion between the gelcoat and the laminate.
For these reasons, transparent gelcoats are as yet only employed for the production of laminates using liquid laminating resins. In prepreg procedures, the mechanical stability of the gelcoat film must be considerably higher since the tackiness of the prepreg impedes handling and, in particular, application thereof on the gelcoat film. It is not generally possible to adjust positioning since the film will tear upon removal of the prepreg from the gelcoat film, or the entire composition might even be wrenched out of the mold. If the gelcoat film is subjected to more intense curing to attain greater stability, however, the laminating time will be insufficient for the formation of laminate layers, particularly in relatively large molds.